1. Field of the Invention
The invention relates to a vehicle drive apparatus that drives wheels by using motor torque.
2. Description of the Related Art
Conventionally, there has been known a four-wheel-drive vehicle configured such that main drive wheels, which are either front wheels or rear wheels, are driven by an internal combustion engine (engine), and auxiliary drive wheels, which are the wheels other than the main drive wheels, are drive by an electric motor. A vehicle drive apparatus mounted in the four-wheel-drive vehicle of this kind is described in, for example, Japanese Patent Application Publication No. 2008-185078 (JP 2008-185078 A).
The vehicle drive apparatus described in JP 2008-185078 A includes an electric motor (motor), a speed reducer, a differential, and a connection-disconnection mechanism. The speed reducer reduces the speed of rotation output from the electric motor. The differential distributes the output from the electric motor to right and left wheels after the speed is reduced by the speed reducer. The connection-disconnection mechanism connects (interconnects) one of a pair of side gears, serving as output members of the differential, and the wheel (left rear wheel) to each other, and disconnects the one of the side gears and the wheel from each other.
The connection-disconnection mechanism includes a first spline gear on the side gear side, a second spline gear on the wheel side, and a synchro sleeve movable in the direction of the axle shafts. When the synchro sleeve meshes with both the first spline gear and the second spline gear in the connection-disconnection mechanism, the side gear and the wheel are connected to each other so that torque is allowed to be transmitted therebetween. The connection-disconnection mechanism is configured such that the side gear and the wheel are disconnected from each other when the mesh of the synchro sleeve with the first and second spline gears is cancelled. The connection-disconnection mechanism includes a triple cone synchromesh mechanism (a three-layer friction transmission member formed of an outer ring, a synchro cone, and an inner ring) that synchronizes the rotation of the first spline gear and the rotation of the second spline gear.
However, in the vehicle drive apparatus described in JP 2008-185078 A, the synchromesh mechanism and a structure (hydraulic circuit) for activating the synchromesh mechanism are complicated, which becomes a factor of increases in cost and weight. Furthermore, if the difference in rotational speed between the first spline gear and the second spline gear is large, the time required to synchronize the spline gears by using friction force of the friction transmission member is long. As a result, the response to the switchover from a disconnected state to a connected state is reduced in some cases.